Art of finishing cut-crystal elements



y 1943- a. w. N. HAWK 2,323,599

ART OF FINISHING CUT CRYSTAL ELEMENT Filed Aug. 17, 1940 Fatented .Fuly 6, 3.943

U N if r caries ART OF FINISHING CUT-CRYSTAL ELEMENTS Henry W. N. Hawk, Merchantville, N. J., assignor to Radio Corporation of America, a'corporation of Delaware Application August 17, 1940, Serial No. 353,031

4 Claims. (01. 171-327) My present invention relates to the art of finishing cut-crystal elements such, for example, as piezoelectric quartz oscillators and resonators.

It is a fact known to every manufacturer of piezo-electric crystals that a thin quartz plate which has been cut to respond to a particular (relatively high) frequency may respond to a diiferent and higher frequency several days or weeks after it has been cut and ground. Such changes in frequency, with time, may be asmuch as several kilocycles in the case of very thin quartz plates. It has accordingly been the practice in this art to so out each crystal element that it will respond originally to a frequency, say, .0l% lower than the desired frequency and to then age it, usually in an oven, for a eriod of from several days to several weeks, until the frequency-response of the'element is stabilized. After aging, the element may be touched up" as by edge-grinding or otherwise, if necessary to cause it to respond to the exact frequency desired.

The aging of high frequency piezoelectric crystal elements is an expensive operation not only because of the additional equipment required and because of the capital tied up in semifinished elements during the aging period, but also because of the necessity of making day-t0- day tests to determine whether frequency stability has been achieved. Of perhaps more importance to thecustomer is the fact that he may have to wait from several days to several weeks for an urgently needed crystal.

It is accordingly an object of my invention to provide an improved method of manufacturing piezoelectric crystal elements and one which ensures the production of highly stable elements without-aging them.

Other objects and advantages of my invention will appear from the following specification when read in connectionwith the accompanying drawing wherein Figures 1 and 2 are sectional views of a quartz piezoelectric element during different steps in the method of my invention.

I attribute frequency drift of the character described to the presence and subsequent disintegration of the Beilby surface layers which are formed on a ground crystal during the grinding process. But little is known of the nature of the surfaces of ground quartz. Sir George Beilbys theory that such surfaces comprise a liquid or amphorous glass-like substance appears to be negatived by the fact that quartz has no noticeable plasticity and great cohesive strength. As to this, see X-ray evidence onthe nature of the surface layers of thin ground quartz crystals secured with the Cauchois spectrograph, by F. R. Hirsh, Jr., and J. W. M. Du Mond, Physical Review, vol. 54, pp. 789-793, November 15, 1938. However this may be, I have observed that a thin quartz plate when aged for several days will acquire a readily removable film comprising a very fine white powder which can be shown, by spectroscopic analysis, to be constituted of pure quartz (silica). I thus believe that whatever is the true nature of Beilby surface layers," the grinding operation, which gives rise to such layers, sets up stresses and strains in the crystal and that the energy thus stored in the crystal is subsequently released over a period of time to cause the disintegration of the said surface layers of the quartz. Obviously, the freeing of the surface layers in the form of a powder by molecular movement has the same effect upon a piezoelectric crystal element as would the removal of an equivalent amount of material by grinding or polishing, that is, it raises its frequency.

As above indicated, it is the grinding or other mechanical disturbance which gives rise to Beilby surface layers in quartz. Hence, wherever in this'speciflcation and in the accompanying claims I refer to a Beilby surface layer" I mean a quartz surface which has been subjected to grinding or other mechanical dis' turbance and hence, presumably, is constituted in whole or in part of broken or fractured elemental crystalline particles.

My present invention contemplates, and its practice provides, the removal of the "Beilby surface layers" from freshly ground crystals, whereby the oscillating characteristics of the crystal.

are immediately stabilized and the necessity for aging is obviated. To this end, I provide each freshly cut crystal with a tough coating constituted of a highly adherent preferably flexible material and subsequently strip the coating together with the Beilby surface layers, whichadhere thereto, from the crystal.

In carrying my invention into efiect, I prefer,

first,to thoroughly wash the crystal as with soap and water, or with carbon tetrachloride or other cleansing material. I then deposit a thin film of silver or other conductive material upon the freshly ground and washed crystal and then, in

order to strengthen and thicken this film, I electroplate it as with nickel or copper so that it may the more readily be stripped from the crystal either in a single piece or in ribbons. The strip ping operation may be Performed with the fingers, or preferably. with a pair of tweezers or the like.

I prefer to deposit the underlying conductive film upon the crystal by the well known "Rochelle salts method" though I may employ the "Brasher methodic -4111c subsequent strengthening or reinforcement of the silver layer by electroplating it should; preferably, proceed until the coating is say .0005" (one-half mil) thick. In some cases, as where there is an imperfection in the underlying conductive film the electroplating process may be continued until the plating is so thick (say 1 mil-of an inch) that it curls and peels 01' its own accord.

In Fig. 1 of the accompanying drawing, Q designates a thin quartz piezoelectric element having a strongly adherent flexible coating thereon comprising a film S of silver which has been reinforced by a layer N of nickel or copper applied by electroplating the underlying silver layer. In Fig, -l, as in Fig. 2, which shows the crystal in the process of being stripped of its reinforced coating, the surface of the crystal is shown by an irregular line B to indicate the so-called "Beilby surface layers" which adhere to the metal coating during the stripping process.

While I have described the method of my invention as applied to a thin quartz piezoelectric element, it will be apparent to those skilled in the art to which my invention appertains that it is likewise applicable to crystalline elements intended for optical and other uses. It is to be understood, therefore, that the disclosure in the above respect is to be interpreted as illustrative and not in a limiting sense, except as required by the prior art and by the spirit of the appended c aims.

What is claimed is:

1. Method of finishing a ground crystalline element which comprises applying to the freshly ground surfaces of said element a strongly adherent coating, and subsequently'stripping said coating.

2. Method of finishing a crystalline element which comprises electroplating said element and subsequently stripping the plating therefrom.

3. Method of finishing a ground crystalline "element which comprises depositing an electrically conductive film on said element, then electroplating said film to strengthen it, and subsequently stripping said strengthened film.

4. Method of stabilizing the frequency of a ground quartz piezoelectric element, said method comprising washing said element, then electroplating said washed element, and subsequently stripping the plating together with the underlying Beilby surfaces" from said element.

HENRY W. N. HAWK. 

